In a friction clutch, for instance for automotive use, a pressure plate for frictionally engaging a clutch disk including clutch facing members is resiliently supported by a clutch cover so as to be urged away from the clutch facing, and the pressure plate is resiliently urged toward the clutch facing by a diaphragm spring when the clutch is engaged. In such a friction clutch, the pressure plate is normally pressed against the clutch facing by the spring force of the diaphragm spring, and the clutch may be disengaged by displacing the diaphragm spring so as to remove the spring force applied to the pressure plate and thereby move the pressure plate away from the clutch facing.
The clutch is disengaged by engaging a central part of the diaphragm spring with a release bearing which is axially actuated by a clutch pedal, and axially moving the central part of the diaphragm spring with the operation of the clutch pedal. The diaphragm spring is deflected away from a back support point defined circumferentially on a back side of the diaphragm spring in a radially middle point thereof as a result, and this releases the pressure applied to the pressure plate.
As the wear of the clutch facing progresses, the position of the pressure plate for engaging the clutch facing shifts toward the flywheel so that the point of force application by the diaphragm spring to the pressure plate also shifts toward the flywheel. As a result, the amount of deflection of the diaphragm spring when engaging the clutch changes with the progress in the wear of the clutch facing, and the orientation of the diaphragm spring changes from its initial state so that the point of force application by the release bearing to the diaphragm spring also changes.
The diaphragm spring is normally provided with a negative spring constant property in the region where the clutch is released so that the pedal force required for disengaging the clutch may be minimized. For technical details of the diaphragm spring having a negative spring constant property, reference should be made to "Paper 1. Friction Clutches and Clutch Control Mechanisms" D. A. Davies, Proc Instn Mech Engrs, 1969-70, Vol 184 Pt 31. The contents of this publication is hereby incorporated in this application by reference. In terms of clutch release force, the change in the point of force application by the release bearing to the diaphragm spring and the configuration of the diaphragm due to the wear of the clutch facing changes the operating point of the diaphragm spring, and this may result in an unacceptable increase in the pedal force required to disengage the clutch.